In the case of spark-ignition internal combustion engine, there is a possibility that an abnormal combustion which is sometimes called “super knock” occurs. This is a phenomenon in which an ignited combustion starts earlier than a proper ignition timing due to an entry of oil components into a combustion chamber while a warming-up is in execution in a state where a temperature of cylinder wall is relatively low. This abnormal combustion occurs only in a low-speed and high-load region. Hence, Patent Literature 1 and Patent Literature 2 disclose a technique of restricting an operating region of the internal combustion engine so as to avoid an actual engine operation in the low-speed and high-load region over which the abnormal combustion generates a problem, when a quantity of oil which intrudes into the cylinder or the like becomes in a state where the abnormal combustion is likely to occur.
However, even in the case that the low-speed and high-load region is defined as a restriction region, and an actual engine operating condition (i.e., torque and rotational speed) is restricted so as not to enter the restriction region as mentioned above; the abnormal combustion can infrequently occur according to various conditions. If the restriction region is set as an excessively large area in order to certainly prevent such an infrequent abnormal combustion, there is a concern that torque shortage is caused at the time of vehicle acceleration or the like. On the other hand, if the restriction region is set as an excessively small area, the abnormal combustion occurs frequently. These both treatments are not favorable.